A vulnerability in the DHCP implementation of Cisco IOS Software and Cisco IOS XE Software could allow an unauthenticated, remote attacker to cause a denial of service (DoS) condition.

The vulnerability occurs during the parsing of crafted DHCP packets. An attacker could exploit this vulnerability by sending crafted DHCP packets to an affected device that has the DHCP server or DHCP relay feature enabled. An exploit could allow the attacker to cause a reload of an affected device.

Cisco has released free software updates that address this vulnerability. There are no workarounds to this vulnerability.

Cisco devices that are running affected Cisco IOS Software or Cisco IOS XE Software with the DHCP server or DHCP relay feature enabled are vulnerable. The DHCP server or DHCP relay feature is not enabled by default. Cisco devices that are configured as DHCP clients are not affected by this vulnerability.

To determine whether a Cisco IOS device or Cisco IOS XE device is configured as a DHCP server, issue the show ip dhcp pool command.

The following example shows a Cisco IOS device that is affected by this vulnerability. The device is vulnerable because the DHCP server feature is enabled and the configured pool has at least one subnet that serves IP addresses:

To determine whether a Cisco IOS device or Cisco IOS XE device is configured as a DHCP relay agent, issue the show run | include helper-address command.

The following example shows a Cisco IOS device that is affected by this vulnerability. The device is vulnerable because the DHCP relay agent feature is enabled, based on the ip helper-address output:

Router#show run | include helper-address ip helper-address 10.1.1.2

To determine the Cisco IOS Software release that is running on a Cisco product, administrators can log in to the device and issue the show version command to display the system banner. The system banner confirms that the device is running Cisco IOS Software by displaying text similar to "Cisco Internetwork Operating System Software" or "Cisco IOS Software." The image name displays in parentheses, followed by "Version" and the Cisco IOS Software release name. Other Cisco devices do not have the show version command or may provide different output.

The following example identifies a Cisco product that is running Cisco IOS Software Release 15.0(1)M1 with an installed image name of C3900-UNIVERSALK9-M:

The DHCP server feature in Cisco IOS Software and Cisco IOS XE Software is a DHCP server implementation that assigns and manages IP version 4 (IPv4) addresses, prefixes, and other information from specified address pools to DHCP clients.
A DHCP relay agent feature in Cisco IOS Software and Cisco IOS XE Software forwards DHCP packets between clients and servers. Relay agents are used to forward requests and replies between clients and servers when they are not on the same physical subnet.

Cisco IOS Software and Cisco IOS XE Software contain a vulnerability that could allow an unauthenticated, remote attacker to cause a DoS condition. An attacker could exploit this vulnerability by sending a crafted request to an affected device that has the DHCP server or DHCP relay agent feature enabled, causing a reload.

The vulnerability is triggered when the affected Cisco IOS device attempts to process a crafted DHCP packet. Valid DHCP packets will not trigger this vulnerability. DHCP packets that the Cisco IOS device forwards (for example, transit DHCP traffic) will not trigger this vulnerability, but the packets forwarded to the DHCP relay agent will trigger the vulnerability.

Cisco IOS devices that are configured as a DHCP version 6 (DHCPv6) server for IP version 6 (IPv6) are not affected by this vulnerability.

This vulnerability has been documented in Cisco bug ID CSCug31561 (registered customers only). This vulnerability has been assigned the Common Vulnerabilities and Exposures (CVE) ID CVE-2013-5475.

When considering software upgrades, customers are advised to consult the Cisco Security Advisories, Responses, and Notices archive at http://www.cisco.com/go/psirt and review subsequent advisories to determine exposure and a complete upgrade solution.

In all cases, customers should ensure that the devices to be upgraded contain sufficient memory and confirm that current hardware and software configurations will continue to be supported properly by the new release. If the information is not clear, customers are advised to contact the Cisco Technical Assistance Center (TAC) or their contracted maintenance providers.

Cisco IOS Software

Each row of the following Cisco IOS Software table corresponds to a Cisco IOS Software train. If a particular train is vulnerable, the earliest releases that contain the fix are listed in the First Fixed Release column. The First Fixed Release for All Advisories in the September 2013 Bundled Publication column lists the earliest possible releases that correct all the published vulnerabilities in the Cisco IOS Software Security Advisory bundled publication. Cisco recommends upgrading to the latest available release where possible.

Cisco has released free software updates that address the vulnerability described in this advisory. Prior to deploying software, customers are advised to consult their maintenance providers or check the software for feature set compatibility and known issues that are specific to their environments.

Customers with Cisco products that are provided or maintained through prior or existing agreements with third-party support organizations, such as Cisco Partners, authorized resellers, or service providers, should contact that organization for assistance with the appropriate course of action.

The effectiveness of any workaround or fix depends on specific customer situations, such as product mix, network topology, traffic behavior, and organizational mission. Because of the variety of affected products and releases, customers should consult their service providers or support organizations to ensure that any applied workaround or fix is the most appropriate in the intended network before it is deployed.

Customers who purchase directly from Cisco but do not hold a Cisco service contract and customers who make purchases through third-party vendors but are unsuccessful in obtaining fixed software through their point of sale should obtain upgrades by contacting the Cisco Technical Assistance Center (TAC):

+1 800 553 2447 (toll free from within North America)

+1 408 526 7209 (toll call from anywhere in the world)

e-mail: tac@cisco.com

Customers should have the product serial number available and be prepared to provide the URL of this advisory as evidence of entitlement to a free upgrade. Customers without service contracts should request free upgrades through the TAC.

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